Getting "undefined reference" in QT Creator when calling a specific function

So I am reading this book: C ++ GUI Programming with Qt 4, Second Edition by Jasmin Blanchette; Mark Summerfield to learn GUI programming. And following the instructions in the book to create a simple spreadsheet application, I get an "undefined" link, an error when I try to compile the Spreadsheet widget.

All errors seem to come from the function Cell *Spreadsheet::cell(int row, int column)

:

Cell *Spreadsheet::cell(int row, int column) const
{
    return static_cast<Cell *>(item(row, column));
}

      

        
        
        
      

    

This is the first function that complains about spreadsheet.cpp

QString Spreadsheet::formula(int row, int column) const {
    Cell *c = cell(row, column);
    if (c) {
        return c->formula();
    } else {
        return "";
    }
}

      

        
        
        
      

    

If the string Cell *c = cell(row, column);

Sends an error: /home/axel/QtSDK/Code/Spreadsheet/spreadsheet.cpp: -1: error: undefined reference to `Cell :: Cell () '

This happens wherever it is called cell(row, column)

. The function itself is defined in the distribution header in the private section as:Cell *cell(int row, int column) const;

Sorry if this seems messy, I am new to C ++ programming.

Here's my .pro file

TEMPLATE = app
CONFIG += console
CONFIG += qt

SOURCES += main.cpp \
           spreadsheet.cpp

HEADERS += \
           spreadsheet.h

      

        
        
        
      

    

If I am missing something, I will add it as soon as I can.

Thanks, Axel